This invention is concerned with a controlled release formulation for the simultaneous delivery of levodopa and carbidopa in the treatment of parkinsonism whereby the adverse reactions and inadequacies often experienced with the administration of standard carbidopa/levodopa combinations are minimized.
SINEMET.RTM. (Merck & Co. Inc., Rahway, N.J.) is the registered trademark for a therapeutic agent useful in the treatment of idiopathic Parkinsonism. It is a combination of levodopa and carbidopa and is provided in tablets of 10 mg carbidopa/100 mg of levodopa; 25 mg of carbidopa/250 mg of levodopa; and 25 mg of carbidopa/100 mg of levodopa. The usual dose is 3 to 4 tablets daily.
Before SINEMET was introduced to the market in 1975, parkinsonism was treated with levodopa by itself. Large doses of levodopa were necessary to adequately control the Parkinson syndrome and severe adverse reactions, especially emesis, were experienced. To minimize these adverse reactions attempts were made to deliver levodopa in a sustained release fashion. In fact there was a product called Brocadopa Temtabs. Several studies failed to show any advantage of the sustained release formulation over a standard preparation. See Eckstein et al., The Lancet, Feb. 24, 1973, page 431 which states at 432, "for the majority of parkinsonians in our study sustained-release levodopa offered no definite advantage over a standard preparation". Also curzon et al., The Lancet, Apr. 7, 1973, page 781, states, "These results suggest there is no practical advantage to be gained by the use of an oral sustained-release preparation of levodopa".
Therapy with SINEMET is widely accepted as the cornerstone in treating idiopathic Parkinson's disease. However, "wearing-off" and "on-off" phenomena have emerged as major problems in the long-term treatment of Parkinson's disease. After two to three years, many patients begin to experience oscillating motor fluctuations which become increasingly disabling. The essential feature is a change from mobility to immobility, which may occur many times a day. Predictable waning of therapeutic effects, following each dose of SINEMET, is known as "wearing-off" and may first occur during stage II-III of the disease. Such response fluctuations occur in 15 to 40% of patients after two to three years of treatment, and in a greater percentage with longer duration of illness. The fluctuations in levodopa levels which accompany SINEMET treatment may in themselves contribute to the development of clinical oscillations.
The clinical manifestations of "on-off" include rapid and unpredictable swings from mobility to immobility. "On" periods can usually be correlated with high or rising plasma levodopa levels and are often associated with distinct, abnormal involuntary movements (dose-related dyskinesias), while "off" periods are commonly but not invariably associated with low or falling plasma-levodopa levels. The relation of "off" periods to low plasma levodopa levels and the observation that the administration of apomorphine during an "off" period may restore function suggests that most such periods are due to cerebral dopamine deficiency. Frequent dosage administration helps to alleviate oscillating clinical responses but dyskinesias and bradykinetic episodes may still occur.
Intravenous levodopa has been used to provide stable plasma levels of 2000 to 5000 ng/ml in advanced parkinsonian patients. This procedure reduces motor oscillations, but optimal response in some patients still include either tremor and bradykinesia or mobility with dyskinesia. High protein meals cause a decline in response without affecting plasma levodopa levels, presumably by inhibiting transport of levodopa into the brain.
The above considerations indicate that a dosage preparation of SINEMET possessing less rapid dissolution properties and providing a more even plasma level profile of levodopa should be efficacious in alleviating some but not all oscillating therapeutic responses.
If the development of clinical fluctuations is promoted by oscillating levodopa levels, a controlled release preparation may also help to prevent the emergence of "wearing-off" and "on-off" phenomena.
Now, with the present invention there is provided a controlled release form of the combination of carbidopa/levodopa designed to obviate or at least alleviate problems associated with the standard combination therapy. Dyskinesias and other central nervous system side effects, and gastrointestinal side effects may be reduced in patients sensitive to high plasma levodopa levels. Patients with oscillating symptoms should respond to the more constant plasma levodopa levels with a more even clinical response. Furthermore, controlled release SINEMET is expected to represent a more convenient dosage form (i.e., allowing for less frequent medication) for many patients who require standard SINEMET four or more times a day. A twice-daily dosage regimen may also be feasible in some patients.